Microporous polyolefin membranes are widely used as membranes for the separation or permselective separation of various substances and as materials for the isolation of various substances. Examples of the use of the membranes may include microfiltration membranes, fuel cell separators, capacitor separators, base materials for functional membranes to allow a novel function to develop by filling pores with a functional material, and battery separators. Among such examples, microporous polyolefin membranes are preferable as separators for lithium-ion batteries widely used in mobile devices such as laptop personal computers, cell phones, and digital cameras. The reasons for this may include the fact that microporous polyolefin membranes have good mechanical strength, electrical insulation performance, and shutdown function.
Conventionally, a carbon material such as non-graphitizable carbon or graphite is used as a material for the negative electrode of lithium-ion batteries. The packing density of such a carbon material (carbon active material) has recently been increasing every year for higher-capacity batteries. Here, a negative electrode using a carbon active material is known to swell during charging. So, a higher packing density of the carbon active material tends to result in much greater swelling of the negative electrode.
On the other hand, alloy negative electrode materials using metals or metalloids (e.g., silicon [Si], tin [Sn]) are being studied as novel negative electrode materials for higher capacity. These alloy negative electrode materials can provide a much higher capacity than carbon active materials. However, these alloy negative electrode materials generally swell much more than carbon active materials.
Patent Document 1 discloses a technology in which in a lithium-ion secondary battery having a high packing density of a carbon active material, the curved corners of the prismatic battery have a lower content of the carbon active material to absorb the strain caused by electrode swelling.
Patent Document 2 discloses a technology in which a combination of simultaneous biaxial stretching with a solvent contained and stretching after solvent extraction provides less change in permeability during compression, absorbing electrode swelling.
In addition, Patent Documents 3 and 4 disclose membranes having a high porosity and a high ion permeability.
Patent Document 1: Japanese Patent Laid-Open No. 2006-278182
Patent Document 2: International Publication No. WO 2006/106783
Patent Document 3: Japanese Patent No. 2961387
Patent Document 4: Japanese Patent Laid-Open No. 2008-88188